The invention relates to a bone joint endoprosthesis, particularly useful as a hip joint prosthesis, in which the body of the joint is held on a shaft by a clamping means.
Federal Republic of Germany published specification DT-OS No. 24 51 275 describes a joint endoprosthesis of this type in which a spigot comprised of a metal alloy is made an integral part of a shaft, and the shaft can be inserted into the femur. The spigot is conically shaped, with a taper ratio between 1:10 and 1:20. The spigot is fitted in a self locking manner into a matching conical bore hole that is formed in a spherically shaped body of a joint. The body of the joint is comprised of sintered ceramic material. This arrangement provides a rigid bond between the body of the joint and the spigot. Furthermore, it avoids the risk of cracking of the body of the joint under too high a load. It has, however, been found that these benefits can best be achieved under particularly favorable conditions and only if both the conical spigot and the matching hole in the body of the joint have been finished to a very high accuracy.
Federal Republic of Germany published specification DT-OS No. 23 18 459 describes a joint endoprosthesis, in which a spherical body of a joint has an internal self-tapping screw thread for fitting on a cylindrical spigot which has been produced as part of the head of a femur. In this way, the screw threaded body of the joint is secured by formlocking so that it cannot be pulled off the spigot in an axial direction. But, unintentional unscrewing of the body of the joint from the spigot is prevented only by clamping forces, and their effect is not sufficiently reliable. The body of the joint could be further secured on the spigot by means of a radially extending screw. But, the drilled hole required for this purpose would unduly weaken the body of the joint and the spigot.
Similar considerations apply to the joint endoprosthesis described in German published specification DT-OS No. 23 40 734, in which a shaft, which can be inserted into the femur, has an externally threaded spigot and where a spherical body of a joint of an oxide ceramic is screwed onto the spigot.